Our long term objective is to understand the molecular mechanisms that control autoimmune diseases. Our immediate focus is on a gene family called Gadd45 (growth-arrest and DNA damage-inducible) which consists of three members,Gadd45a, Gadd45b, and Gadd45g. Gadd45a was shown to be involved in regulating homeostasis of T cells and lack of Gadd45a was known to cause lupus. The role of the other two family members, Gadd45b and Gadd45g, in autoimmune diseases is not clear. In Th1 cells, Gadd45b and Gadd45g, but not Gadd45a, are induced by TCP signaling or IL-12 and IL-18. We have found that the lack of Gadd45b and Gadd45g results in a drastically reduced number of Th1 cells against Listeria monocytogenes. Expecting low numbers of Th1 cells, we were surprised to see that Gadd45b deletion resulted in exacerbated experimental allergic encephalomyelitis (EAE) with more severe clinical signs, a prolonged disease course and increased autoreactive Th1 cells in the inflamed CNS. Gadd45b deletion also resulted in enlarged spleens in older mice. Gadd45b/Gadd45g double-deficiency further aggravated this phenotype and resulted in greatly enlarged spleens in older mice compared to Gadd45b single deletion. The enlargement of spleens was due to the accumulation of CD4+ T cells with an activated phenotype and B cells. Our data suggest that Gadd45b and Gadd45g play a synergistic role in regulating activated CD4+ T cells. In addition, we found that Gadd45b and Gadd45g inhibited proliferation and were required for apoptosis of activated CD4+ T cells. In this proposal we are testing the hypothesis that Gadd45 protein family members Gadd45b and Gadd45g are important negative regulators of autoimmunity. Specifically we plan to: 1. provide definitive proof that Gadd45b and Gadd45g coordinately regulate autoimmune diseases, 2. determine if Gadd45b and Gadd45g are critical for the control of T cell proliferation and apoptosis in EAE, and 3. study molecular mechanisms that regulate the proliferation and apoptosis of Th1 cells by Gadd45b and Gadd45g. Regulation of proliferation and apoptosis in peripheral effector CD4+ T cells by Gadd45 family of molecules provides a new regulatory mechanism for autoimmunity. Relevance: This study will lead to the development of novel strategies targeting these molecules to treat or prevent autoimmune diseases. This study will also reveal new disease markers for autoimmune diseases.